Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 6, 2016
Valid: July 7-11, 2016 (Thursday-Monday)

Summary:

Thursday and Friday remain days of interest for the northern Mid-Atlantic, with the presence of a mid-level ridge and lee trough bringing temperatures along the I-95 Corridor well above average, into the low to mid 90s °F. Intense sunshine and short, westerly back trajectories both days will promote rapidly rising ozone along the I-95 Corridor, likely resulting in ozone exceedances. The main forecast question will be the possible development of diurnal clouds and thunderstorms, with ample regional heat and moisture providing substantial low level instability. It appears that the I-95 Corridor will remain dry through most of Thursday, allowing for a full day of ample ozone production. The chance for an ozone exceedance will remain High on Thursday as a result for locations along and east of the I-95 Corridor. There is more support for clouds and convection on Friday, due to more intense shortwave activity aloft and an approaching backdoor frontal boundary approaching the northern Mid-Atlantic. Ozone exceedances are still expected in the vicinity of the I-95 Corridor given abundant afternoon sunshine ahead of the precipitation, but they will likely be more isolated in nature The chance for an ozone exceedance will drop slightly to Appreciable as a result. On Saturday, afternoon sunshine will be bookended by two frontal passages, both of which will bring clouds and scattered precipitation to the northern and central Mid-Atlantic. A noticeable faster, albeit westerly, transport pattern aloft will push pollutants to the east of the I-95 Corridor. The chance for an ozone exceedance will fall to Marginal on Saturday for these locations. The northern and central Mid-Atlantic will experience an air mass change on Sunday behind the second cold front, which will push southward toward the VA/NC border. Back trajectories will shift northwesterly and presumably begin to ventilate the I-95 Corridor. Therefore, the chance for an ozone exceedance will drop to Slight on Sunday. On Monday, surface high pressure will move into western PA, promoting clear skies and subsiding winds across the northern Mid-Atlantic. A fast, northwesterly transport pattern will remain aloft, tempering rising ozone levels along the I-95 Corridor. The chance for an ozone exceedance on Monday will rise to Marginal for the I-95 Corridor.

Discussion:

The weather forecast models are in general agreement through most of the medium range, but are still struggling to reach consensus with the development and track of several shortwave impulses aloft during the first half of the forecast period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A smattering of shortwaves will move into a weak upper level trough across the central and southern Mid-Atlantic on Thursday. A mid-level ridge continues to envelop the eastern U.S., and will remain in place through Friday. On Friday, a slightly stronger shortwave will traverse the northern Mid-Atlantic through 12Z ahead of a potent upper level trough developing across the Great Lakes. This shortwave will pull a backdoor frontal boundary southward across NY around 12Z towards the northern Mid-Atlantic, but its exact placement remains a question. The trough will slowly progress eastward on Saturday, with its axis only reaching the western edge of the Mid-Atlantic by 18Z Saturday. In addition to suppressing the mid-level ridge, this approaching trough will push the backdoor frontal boundary into the central Mid-Atlantic on Saturday. It is worth noting that the NAM is showing a much broader and weaker trough than the GFS and EC, which both resolve a more amplified trough. By 06Z Sunday, the trough will move directly over the northern Mid-Atlantic, pulling a second, stronger cold front eastward across the forecast region. Beginning 18Z, the trough will begin to lift northeastward toward ME/Canadian Maritimes. The Mid-Atlantic will start to feel the impacts of an upper level ridge amplifying over the Northern Plains on Monday, with its eastern edge reaching the forecast region by 18Z.

Thursday is a day of interest for the Mid-Atlantic, with the continued effects of a mid-level ridge and a persistent surface lee trough driving temperatures well above average across the northern Mid-Atlantic. Excessive heat warnings have been issued by NWS for locations across PHL, western NJ, and northern DE, and will in effect through Friday. This intense heat will be conducive to ozone production along the I-95 Corridor. 06Z GFS back trajectories remain short and westerly, originating from the Ohio River Valley, the historical source region for NOx. Surface winds will be relatively light and southwesterly, which will act to push pollutants up the I-95 Corridor into PHL and NJ. How high ozone climbs on Thursday will partially depend on how quickly the air mass in place modifies today, but given widespread titration and climbing temperatures across the Mid-Atlantic this morning, it seems that rapid ozone formation this afternoon is likely. The only potential limiting factor Thursday will be the development of diurnal clouds and convection across the northern and central Mid-Atlantic beginning around 18Z. There will be ample moisture and convergence along the lee trough coupled with orographic lifting west of the Corridor may be sufficient to trigger convection. The 03Z SREF and 06Z 4km NAM keep afternoon precipitation just west of the I-95 Corridor through 00Z Friday, while the 06Z 12 km NAM and 13 km GFS develop isolated precipitation across I-95 beginning 21Z Thursday. Several of the 06Z air quality model runs have backed off slightly in comparison to yesterday’s runs, likely in response to this precipitation. The BAMS and NCDENR models now show mid to upper Moderate ozone along the I-95 Corridor for Thursday. Only the NOAA model resolves ozone exceedances for tomorrow, showing Moderate ozone across DC, northern DE, PHL, and most of NJ (with a patch of Unhealthy ozone in central NJ). The BAMS and NCDNER air quality model solutions may verify if widespread enough convection does indeed form in the afternoon, but rapid ozone production on Thursday makes another day of at least isolated ozone exceedances more likely. Therefore, the chance for an ozone exceedance will remain High on Thursday for areas along and east of the I-95 Corridor.

Friday’s forecast is also complicated by questions regarding diurnal convection, as well as the potential effects of a backdoor frontal boundary approaching the northern Mid-Atlantic around 12Z. The NAM keeps the majority of afternoon clouds and thunderstorms to the north of the forecast region through 00Z Saturday, while the GFS and EC show widespread precipitation developing across the northern Mid-Atlantic beginning 18Z Friday. The 06Z Hi-Res appears to favor the GFS/EC solution at this point in time. However, the fully convection-allowing models (the 4 km NMM and ARW) are less aggressive with convection. It appears that stronger shortwave support aloft (in comparison to Thursday), paired with lifting ahead of the nearby backdoor stationary front, might provide the lift that was missing on Thursday to spark late day convection and thunderstorms. That said, another day of sweltering conditions and a short westerly transport aloft may be sufficient for abundant ozone formation along the I-95 Corridor. Additionally, the regional air mass is expected to be thoroughly modified from two days of intense sunshine and potential ozone exceedances. Despite these factors, the NCDENR model brings most of the I-95 Corridor into the low Moderate range for ozone on Friday. It is worth mentioning, however, that there have been concerns this week with the NCDENR model underdoing ozone levels due to anomalously low EGU emissions during this same week in 2005, which is used as the base year for its modeling platform. The BAMS models keep most of the I-95 Corridor and its surrounding areas in the upper Moderate range, and develop pockets of USG ozone across northern NJ, PHL, and DC. Given the increased likelihood of diurnal clouds and precipitation, it seems likely that ozone exceedance seen on Friday will be more isolated in nature compared to Wednesday and Thursday. The chance for an ozone exceedance will drop slightly to Appreciable on Friday as a result.

On Saturday, ample afternoon sunshine will be bookended by two frontal passages, which will trigger clouds and precipitation across the northern and central Mid-Atlantic. The first frontal passage will occur in the morning when the backdoor front from Friday swings down across the northern half of the forecast region, moving just offshore by roughly 09Z. The second frontal passage will occur in the late afternoon, as a second, stronger cold front pushes eastward across the region, reaching the I-95 Corridor by 00Z. Back trajectories for the I-95 Corridor remain westerly, but are much faster, which will likely push pollutants just east of I-95. Additionally, the mid-level ridge will be flattened out by an approaching upper level trough. As a result, the BAMS models bring most of the I-95 Corridor into the Moderate range for Friday, with the only ozone exceedances being seen along the eastern NJ shore. The chance for an ozone exceedance will fall to Marginal for locations just east of the I-95 Corridor.

The northern and central Mid-Atlantic will experience an air mass change on Sunday behind the second cold front, which will push southward towards the VA/NC border by 12Z. Mid-level cooling and a shift to relatively fast, northwesterly transport should both knock down temperatures a few degrees and temper rising ozone along the I-95 Corridor. The BAMS model strongly reflects this, showing ozone levels well into the Good range across the entire Mid-Atlantic. Though this solution might be a little extreme, it certainly seems likely that synoptic setup in place on Sunday will allow for gradual atmospheric ventilation and cleaning throughout the forecast region. The chance for an ozone exceedance will drop to Slight on Sunday as a result.

On Monday, surface high pressure will build into western PA ahead of an approaching upper level trough. Ample afternoon sunshine and subsiding winds across the northern Mid-Atlantic will allow ozone levels to rise along the I-95 Corridor. Back trajectories for the I-95 Corridor, however, remain fast and northwesterly, which should have the same moderating effect as it did on Sunday. At this point in time, the chance for an ozone exceedance will rise to Marginal on Sunday in response to effects of nearby high pressure.

-Brown/Ryan